Bashar al-Assad's military machine is on the brink of logistical meltdown and collapse, because it lacks petrol and food, and is having problems resupplying its soldiers, according to a Syrian general who has defected to the opposition.

Much has been made of the Syrian military's supposed superiority over the opposition, but General Mohammad Al-Zobi told the Guardian: "The benzine is nearly finished. They are running out of rockets. There is scarcely any bread or water for the soldiers."

Zobi defected two months ago alongside his air force colleague General Saed Shawamra. They slipped out from Tiftanaz airbase in the middle of the night. From the city of Idlib they crept across the border to Turkey. On Wednesday they crossed back into Syria, their mission now to finish off the revolution against Assad.

The men, from Dera'a province, are among around 100 senior military commanders who have joined Syria's rebels, appalled – they say – by Assad's brutal war against his own population. According to Zobi, the embattled Syrian regime can last "one or two months at most". "After that Assad will leave Syria. He'll go to Russia or maybe Iran," Zobi predicted, sitting in a village in rustic northern Syria, close to the Turkish border.

It is, of course, in the interests of the rebels to paint a picture of a crumbling regime on the brink of collapse, but it chimes with the view of General Robert Mood, the former head of the UN monitoring mission in Syria, who told Reuters on Friday: "In my opinion it is only a matter of time before a regime that is using such heavy military power and disproportional violence against the civilian population is going to fall.

"Every time there are 15 people killed in a village, 500 additional sympathisers are mobilised, roughly 100 of whom are fighters," Mood said.

But Mood suggested it may take more than a few months for Assad's regime to fall. "In the short term it may very well be possible for him to [hold on], because the military capabilities of the Syrian army are much, much stronger than those of the opposition," he said.

"The minute you see larger military formations leaving the ranks of the government to join the opposition, then that is when it starts accelerating … This could last for months or even years," he said.

Over the past few months the Free Syrian Army (FSA) has taken control of large swaths of the countryside, carving out a mini-empire in the north and east. The regime is largely confined to urban areas.

This new geographical reality has given opposition fighters the capacity to degrade the military's creaking supply chain. The FSA has ambushed army petrol tankers. And it has shot up trucks laden with provisions, according to Zobi. Earlier this week fighters also attacked an armoured column sent from Idlib to reinforce government positions inside Aleppo, the city partially seized by the rebels last Friday.

The rebel commander in charge of Idlib province, Mohammad Issa, said his fighters were camped outside regime-held Idlib but had decided not to enter the city. They were anxious to avoid the fate of Bab al-Hawr, the district of Homs pulverised by Assad's forces. "We are besieging the army. The army isn't besieging the FSA. We know all the movements of the regime army," Issa said confidently.

The two generals had been in charge of helicopters at the Tiftanaz base, outside Idlib. Because of rebel attacks on supply routes, the garrison was now forced to fly in food and ammunition by plane from Aleppo, they said. It was a similar picture in other army bases, increasingly vulnerable and cut off, Zobi suggested.

Assad's greatest advantage over his lightly armed opponents comes from the sky. Syria has 150 Soviet-built helicopters, including M8 and M17 troop transporters, capable of transporting 24 soldiers each. Russia had also delivered "five or six new helicopters" over the past month, the generals said.

But the president's most lethal weapon is his notorious M25 helicopter gunship. Syria has 22 of them, stationed in pairs at every airbase across the country, according to Zobi. They are remorseless killing machines able to fire 64 rockets on each mission and 2,000 machine gun rounds of varying calibres. They can stay in the air for four and a half hours.

"You can't shoot them down. It's impossible. They fly at an altitude of 4.5kms, above the range of a Kalashnikov," Zobi said. The gunships have a crew of four: two pilots, a gunner and an engineer.

The general added: "We defected after our superior gave us an unambiguous order to shoot everything on the ground."

"We didn't want to kill our own civilians," Shawamra explained.

The officers' unprecedented insider account appears to be corroborated by events inside Aleppo, where 34 people were killed on Thursday, according to the British-based Syrian Observatory for Human Rights.

Residents reported seeing two M25s: one above Salheddin in the south of the city, the other hovering over Sakhour in the east. The gunships are likely to play a key role in an imminent government offensive to drive the rebels out.

The generals also painted a portrait of a demoralised Assad army. Some 30% of the president's soldiers had deserted, they said, almost all of them Sunni Muslims. Those who remained were Alawites – members of Assad's ruling sect – and members of other Shia groups, convinced that Syria was descending into a Rwandan-style sectarian war.

Spirits were low, Zobi added. "They [the soldiers] are nervous, tired and hungry. Most have been on their bases for three or four months," he said.

"They haven't seen their families. When they call their wives there is a spy listening on the line. Many soldiers are in military prisons.

"And anyone who deserts and who is caught is immediately shot."

Other defectors who had made their way to northern Syria confirmed the generals' bleak view, and said the rank and file in Assad's army wanted out. "The soldiers want to leave the army but they are very afraid. They want a no-fly zone to protect them. They are afraid they will themselves be shelled," Major Salem said. He declined to give his second name, and said his family was in a regime area.

Salem defected on 17 December 2011. He spent five months battling the FSA in Homs, including in the old city. "We had orders to shoot the hospital and kill civilians. Government forces shot everybody. No difference whether it was a child, woman, or old man," he said. "I left because I had to defend my people, my family."

Mortars, artillery, planes, and tanks had all been used to punish Homs, he said, one of the epicentres of Syria's 16-month rebellion. He also claimed that nerve gas had been used in the Al-Rastan district.

"I was still in the army at the time. It was dropped from a plane," he alleged.

Kardash recalled how he had worked as a driver for Assad's personal pilot, a high-ranking officer in air force intelligence based at Damascus international airport.

Kardash – a Sunni, who was imprisoned in 2010 – said that when the fighting started, Syria's top brass swapped their Mercedes staff cars for Toyota Prado 4x4s. He said he had glimpsed Syria's president on "14 or 15 occasions" getting into his Russian Yak jet at Damascus airport. "He is a donkey. He's killing his own people," Kardash said.

The jet was lavishly equipped with a kitchen, comfortable chairs, and presidential phone, he added. Assad also owned three French Dolphin 117 civilian helicopters, used to ferry him around.

The president's current whereabouts are unknown. He has not been seen since last week's devastating bomb attack in Damascus killed four members of his military-security command. Some observers believe that Assad is unlikely to flee the country, and will fight on until the end.

But General Shawamra demurred. He sketched out Assad's likely escape route: a plane to the impregnable Russian naval base in Tartus, Syria's port city on the Mediterranean, and from there to Moscow or somewhere else. "The Russians have so many guards. Nobody could shoot their way past them," he reasoned.

And then there is the economic aspect fuelling discontent and mutiny inside Syria's military structure. Shawamra said that after 25 arduous years as a career staff officer he had been earning a salary of 31,000 dinars a month when he ran away – a meagre £300. Kardash said he had been getting just £140 a month. "In half a month it's all gone," he joked.

And what of the supposed FSA attack on the national security headquarters in Damascus last week? The blast killed the president's brother-in-law, Assef Shawkat, as well as his national security chief, Hisham Bakhtiar, who died of wounds several days later. Syrian state TV quickly released news of the explosion – an atypical move that has fuelled numerous conspiracy theories inside Syria, including that the officers were executed after a palace coup against Assad that went wrong.

General Zobi, however, said the FSA had indeed blown the commanders up. The plot loosely resembled the failed attempt by Von Stauffenberg to blow up Adolf Hitler, he suggested. "We used 15 kilos of TNT," he said. "It was smuggled inside in small amounts. Some of it was under the table and the rest was in a room next door."

And was there a suicide bomber? "No," the general said. "We detonated it remotely."